FIG. 8 shows a schematic cross-sectional view of a conventional cathode-ray tube. A cathode-ray tube 91 comprises a glass bulb 92, a neck portion 95, an electron gun 93 contained in the neck portion 95, and a phosphor screen 94.
FIG. 9 shows an enlarged cross-sectional view of an indirectly heated cathode used in an electron gun. An indirectly heated cathode 1 comprises a sleeve 6, a heater 2 inserted into the sleeve 6, a metal cover 7 located on the top of the sleeve 6, and an emissive material 8 provided on the metal cover 7. The heater 2 is formed of a metal wire 3, an alumina insulating layer 4, and a dark layer 5 having a thickness of about 3 .mu.m. The metal wire 3 is a coiled base metal of the heater, which is made of tungsten or rhenium-tungsten. The alumina insulating layer 4 is made of alumina and the like and is formed on the surface of the metal wire 3. The dark layer 5 is made of tungsten and alumina and is formed on the alumina insulating layer 4.
The alumina insulating layer 4 is formed of an inorganic porous film. The alumina insulating layer 4 is required to have a uniform thickness and a uniform filling rate in order to prevent the occurrence of cracks caused by the concentration of thermal stress when electricity is conducted in the heater and to provide an appropriate elasticity to the heater.
Generally, electrophoresis (electrodeposition) is employed for forming an alumina insulating layer. In the case of conducting the electrodeposition of alumina powder, impurity electrolyte adhering to alumina particles is removed by dipping and washing the alumina particles in pure water. Unexamined Japanese Patent Application Tokkai Sho 59-200798 discloses that powder is washed with pure water or deionized water until the conductance of the water used for washing the powder decreases to a predetermined value or less.
As a method for obtaining a uniform filling rate of an alumina insulating layer, for example, Unexamined Japanese Patent Application Tokkai Hei 4-127022 discloses a technique for making ceramic particles adhere to the portion between adjacent coiled metal wires by conducting a pulse electrodeposition and a lamination electrodeposition successively so that the ceramic particles are distributed uniformly.
However, the technique described in the above-mentioned Tokkai Hei 4-127022 complicates the manufacturing processes. Therefore, there is a problem that the technique is not suitable for mass production.
In addition, there is the following problem in the technique of washing powders as disclosed in the above-mentioned Tokkai Sho 59-200798. That is, conventionally, in the case of washing powders, supernatant water is replaced after the first wash with a centrifugal machine. The washing and the replacement of supernatant water are repeated until a predetermined conductance is obtained from the supernatant water. When the washing according to this method was conducted using ordinary-temperature water (20.degree. C.), it took about 24 hours. Further, since impurities present on the surfaces of or inside powders were not removed sufficiently, a lot of impurities are eluted into a suspension for electrodeposition. Therefore, the conductance of the suspension becomes high in a short period. Thus, the electrophoretic property of the alumina particles decreases. For instance, a suspension for electrodeposition having a conductance of 6.25 .mu.S/cm had a conductance of 12.5 .mu.S/cm after one week of elapsed time.
Generally, when the surfaces of alumina powders are in bad condition (an inert condition caused by the adhesion of impurities to the surfaces), the thickness of an alumina insulating layer is not uniform. As a result, deformation, cracks, insulation failure or the like occurs, which causes a decrease in manufacturing yield. When the surfaces of the alumina particles are in bad condition, the electrophoretic property of the alumina particles decreases. Therefore, the possibility of the formation of an uneven alumina insulating layer becomes high, even if the method of electrodeposition is adjusted. In contrast, when the surfaces of alumina particles are made to be in good condition (in an active state), the electrophoretic property of the alumina particles improves, thus obtaining a uniform alumina insulating layer with a simple method.
When the surfaces of alumina particles are in bad condition, the alteration of a suspension for electrodeposition that is used for electrodepositing alumina particles to the heater base metal (the decrease in the electrophoretic property of the alumina particles) is accelerated. Thus, it becomes difficult to manufacture heaters stably.
As described above, when the surfaces of alumina particles are in bad condition, many of the manufactured heaters have an alumina insulating layer that is not uniform in thickness and filling rate. Consequently, since the thermal stress is concentrated at the uneven parts, the occurrence of cracks or the deformation is accelerated, thus shortening the life of the heater. In addition, a lot of work is required for examining and sorting heaters with such an uneven alumina insulating layer. As a result, the productivity decreases considerably.